Investigations of biopsy material from human kidneys with different forms of glomerulonephritis (n = 1,240) and with diabetic glomerulosclerosis (n = 406) performed in order to find changes caused by hyperperfusion of the kidney tissue gave the following results: (1) Hyperperfusion injury occurs in the different forms of glomerulonephritis with varying frequency. It was rarely found in immunologically negative mesangioproliferative glomerulonephritis. The highest incidence was found in patients with membranoproliferative glomerulonephritis type I. (2) Hyperperfusion injury was also found in kidneys with diabetic glomerulosclerosis. The frequency of this finding increased with the degree of the diabetic changes. (3) The hyperperfusion injury was seen as a complication of glomerulonephritis or diabetic glomerulosclerosis only when the patient clinically had developed malignant hypertension and when the serum creatinine level was elevated, a sign of compensated retention. (4) In patients with glomerulonephritis, the hyperperfusion changes occurred more frequently in males than in females. Diabetic glomerulosclerosis was complicated by hyperperfusion injury with the same frequency in both sexes. (5) Patients with hyperperfusion changes of the kidneys always excrete large amounts of protein in the urine. (6) Hyperperfusion changes occur first in the juxtamedullary glomeruli. The intermediate glomeruli are affected later and the subcapsular glomeruli last.